A basic calculator app in Java Swing.

Calculator.java

import javax.swing.*;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.math.MathContext;

enum Calculation {
	ADDITION,
	SUBTRACTION,
	MULTIPLICATION,
	DIVISION
}

class Window {
	private JFrame frame;
	private JLabel labelA;
	private JLabel labelB;
	private JLabel result;
	private JTextField textFieldA;
	private JTextField textFieldB;
	private JButton plusButton;
	private JButton minusButton;
	private JButton multiplyButton;
	private JButton divideButton;
	
	public Window() {
		frame = new JFrame("Calculator");
		labelA = new JLabel("A: ");
		labelB = new JLabel("B: ");
		result = new JLabel("");
		textFieldA = new JTextField();
		textFieldB = new JTextField();
		plusButton = new JButton("+");
		minusButton = new JButton("--");
		multiplyButton = new JButton("x");
		divideButton = new JButton("/");
		
		frame.setBounds(483, 184, 500, 400);
		labelA.setBounds(170, 35, 30, 30);
		labelB.setBounds(170, 85, 30, 30);
		textFieldA.setBounds(200, 35, 100, 30);
		textFieldB.setBounds(200, 85, 100, 30);
		plusButton.setBounds(75, 175, 50, 50);
		minusButton.setBounds(175, 175, 50, 50);
		multiplyButton.setBounds(275, 175, 50, 50);
		divideButton.setBounds(375, 175, 50, 50);
		result.setBounds(50, 270, 400, 30);
		
		plusButton.addActionListener((actionEvent -> calculate(Calculation.ADDITION)));
		minusButton.addActionListener(actionEvent -> calculate(Calculation.SUBTRACTION));
		multiplyButton.addActionListener(actionEvent -> calculate(Calculation.MULTIPLICATION));
		divideButton.addActionListener(actionEvent -> calculate(Calculation.DIVISION));
		
		frame.add(labelA);
		frame.add(labelB);
		frame.add(textFieldA);
		frame.add(textFieldB);
		frame.add(plusButton);
		frame.add(minusButton);
		frame.add(multiplyButton);
		frame.add(divideButton);
		frame.add(result);
		frame.setLayout(null);
		frame.setVisible(true);
	}
	
	public void calculate(Calculation calculation) {
		BigInteger a;
		BigInteger b;
		
		try {
			a = new BigInteger(textFieldA.getText());
			b = new BigInteger(textFieldB.getText());
			
			switch (calculation) {
				case ADDITION:
					result.setText(String.format("%d + %d = %d", a, b, (a.add(b))));
					break;
				case SUBTRACTION:
					result.setText(String.format("%d - %d = %d", a, b, (a.subtract(b))));
					break;
				case MULTIPLICATION:
					result.setText(String.format("%d * %d = %d", a, b, (a.multiply(b))));
					break;
				case DIVISION:
					result.setText(String.format("%d / %d = ", a, b) + new BigDecimal(a).divide(new BigDecimal(b), new MathContext(2)));
					break;
			}
		} catch (Exception e) {
			result.setText("Error!");
		}
	}
}

public class Calculator {
	public static void main(String[] args) {
		new Window();
	}
}